Transport and fate of microorganisms in soils with preferential flow under different solution chemistry conditions

Laboratory and numerical studies were conducted to investigate the transport and fate of Escherichia coli D21g and coliphage phi X174 in saturated soils with preferential flow under different solution ionic strength (IS=1, 5, 20, and 100 mM) conditions. Preferential flow systems were created by embedding a coarse-sand lens (710 mu m) into a finer matrix sand (120 mu m). Complementary transport experiments were conducted in homogeneous sand columns to identify controlling transport and retention processes, and to independently determine model parameters for numerical simulations in the heterogeneous experiments. Results from homogeneous and heterogeneous transport experiments demonstrate that retention of E. coli D21g and phi X174 increased with IS, while the effect on E. coli D21g in finer sand was much greater than in coarse sand. This microbe transport behavior was well described by numerical simulations. The importance of preferential flow on microbe transport was found to be enhanced at higher IS, even though the overall transport decreased. However, the contribution of preferential flow was much higher for E. coli D21g than phi X174. Deposition profiles revealed significant cell retention at the interface of the coarse-sand lens and the fine-sand matrix as a result of mass transfer. Cell release from the preferential flow system with a reduction of solution IS exhibited multipulse breakthrough behavior that was strongly dependent on the initial amount of cell retention, especially at the lens-matrix interface.